10. Post Transl. Modification Flashcards
Give an example of a protein that is constitutively secreted?
Collagen
Give an example of a protein that has regulated secretion.
Insulin
Which cell secretes collagen in connective tissue?
Fibroblasts
What is the basic unit of a collagen fibre?
Tropocollagen
Describe the structure of tropocollagen.
3 alpha polypeptide chains arranged in a triple helix.
What motif is characteristic of collagen?
Gly-X-Y
Why is glycine so important in collagen structure?
It is the only amino acid with a side chain small enough to fit in the middle of the helix.
What are the properties of collagen?
High tensile strength, non-compressible, non-extensible
In addition to glycine, what is most commonly found in the X and Y positions in the chain?
Proline or hydroxyproline
What type of bonds are present in tropocollagen to stabilise the structure?
Hydrogen bonds between alpha chains
What types of alpha chains are present in type 1 collagen?
2 alpha 1 chains
1 alpha 2 chains
Where is type 1 collagen found?
Skin, tendons, ligaments and bone.
90% of body collagen
Where is type 2 collagen found?
Cartilage - hyaline and elastic
Fibrocartilage has both types 1 and 2
Before entering the ER, what is the protein chain called?
Prepro alpha chain
What happens to the prepro alpha chain as it enters the ER?
The signal peptide at the N terminal is cleaved by signal peptidase to leave pro-alpha chains
What 3 modifications to the procollagen chains take place in the ER?
- Hydroxylation of selected proline and lysine residues
- Addition of N-linked oligosaccharides
- Addition of galactose to hydroxylysine residues
After modification to the pro-alpha chains, how do the 3 chains align and attach to eachother?
Disulphide bonds are formed at the C terminal region between the alpha chains
What exists at the N and C terminals of each alpha chain of the pro-collagen molecules?
There are extra amino acids that DO NOT form a triple helix
What modifications does the procollagen undergo in the GOLGI?
O-linked oligosaccharide chains are completed by the addition of glucose
What happens to the procollagen molecule after leaving the golgi before it can become a tropocollagen unit.
It leaves the golgi in a transport vesicle and is exocytosed from the cell. The remaining N and C terminal sections are removed by procollagen peptidases to produce a tropocollagen unit.
Which terminal of the procollagen contains disulphide bonds?
C terminal
Where are procollagen peptidases expressed, why is this so important?
Only expressed extracellularly, prevent tropocollagen fibril formation from occurring inside cells.
How is a collagen fibril formed?
Lateral association of tropocollagen with covalent cross-linking produces a collagen fibril.
How does a collagen fibril become a fibre and why are they striped in appearance?
Aggregation of fibrils to form a fibre. There are gaps between the individual collagen molecules which makes it stripey.
What enzyme is responsible for the covalent bonds involved in tropocollagen cross-linking?
Lysyl oxidase - extracellular
What are cofactors for lysyl oxidase?
Vitamin B6
Cu 2+ ions
What disease is linked to lysyl oxidase deficiency or mutation in type V collagen?
Ehlers - Danlos Syndrome (EDS)
Which enzyme is responsible for hydroxylation or pro-alpha chains in the ER?
Prolyl hydroxylase
What are the cofactors of prolyl hydroxylase?
Vitamin C and Fe2+ ions
What is the importance of hydroxylation of procollagen in the ER?
It allows increased H+ bonding to stabilise the triple helix
How can scurvy affect collagen synthesis?
Lack of Vitamin C prevents activity of prolyl hydroxylase, so protein contains less hydroxyproline.
Weak tropocollagen triples helices as reduced hydrogen bonding.
